1. Field of the Invention
The present invention relates to a slip control system for an automotive vehicle.
2. Description of the Related Art
Recently, an increasing number of automotive vehicles are loaded with a slip control system, as usually called "ABS system", which is arranged to prevent the wheels from being locked at the moment of braking (for example, Japanese Utility Model Laid-open Publication No. 57-130,754).
In order to implement the control of the ABS system, i.e. to control the braking force, there may be at least two modes for controlling the braking force, a first mode being so adapted as to control the activity to reduce the braking force, and a second mode being so adapted as to control the activity to elevate the braking force. In addition, a mode may added as a third mode which is so adapted as to control the activity to sustain the braking force.
It is noted herein that whether the wheel is locked can usually be determined on the basis of a slip value determined from an assumed vehicle speed to be given theoretically and an actual wheel speed. Hence, the determination if the control of the braking force should be started can be implemented by determining if the slip value exceeds a predetermined threshold value.
Further, it can be noted that the magnitude of deceleration indicates a behavior of the wheels with considerable accuracy and that the magnitude of deceleration can be determined with ease and with accuracy by differentiating the wheel speeds detected by wheel speed sensors. Hence, if the ABS control should be started can be determined by determining if the magnitude of deceleration of the wheel exceeds a predetermined threshold value. Japanese Patent Laid-open Publication (kokai) No. 61-24,220 discloses an example that takes advantage of the magnitude of deceleration of the wheel for the ABS control. This publication proposes that if the control of the activity to reduce the braking force has been finished is decided by determining if the magnitude of deceleration of the wheel exceeds the predetermined threshold value.
On the other hand, strong demands in the ABS control have recently been made to shorten a distance required upon braking the wheels, i.e. a braking distance. In order to shorten the braking distance, it is disadvantageous to cause the braking force to be reduced to an unnecessary extent by the ABS control. Hence, it is considered to start the ABS control from the state in which the braking force is sustained.
It can be noted, however, that, if the ABS control would be always started from the state in which the braking force is sustained, the wheels are caused to be locked more or less when the wheels are brought into a state in which they are very likely to be locked, for example, at the time of rapid braking.